Here's the thread from when I made my first aid kit for the first time:
http://rabbitsonline.net/view_topic.php?id=75235&forum_id=48 - lots of input from numerous people.
I consider probiotics, simethicone, some sort of pain med (preferably metacam if you can get some from your vet), Critical Care and at least two syringes (1 cc for meds and 3 cc or larger for syringe feeding; interestingly, I seem to have better luck with syringe feeding when using a 3 cc syringe than with a 6 cc syringe) to be musts - stasis is the most common emergency situation with bunnies and those things are all key. Some sort of prescription gut motility drug like metoclopramide/cisapride or whatever the main one is depending on the country you live in is also *really* useful, but a bit of a luxury... it generally takes a run-in with stasis to end up acquiring a stash of it from your vet.
Beyond stasis, some other useful things are vet wrap, gauze pads, a rabbit-safe heating pad (which can be as cheap and basic as a large sock filled with uncooked rice that can be warmed in the microwave), neosporin *without* pain reliever or better yet, Vetericyn (non-prescription/animal specific wound treatment; it's carried by some vets, some feed stores and sold online), liquid bandaid (something I'm never without just for my own use, heh - it's a "must have" when you have yet-to-bond-to-you sugar gliders who like to bite your finger tips and draw blood), canned pumpkin (PLAIN, not pie filling), a jar or two of preservative-free fruit or veggie flavored baby food (preferably one with a lower sugar content - they're not advertised as such, but among different fruit flavors of the same brand, I've noticed that some have twice the sugar of others).
I'm probably missing a couple handy things off the top of my head... I sell first aid supplies on my sugar-glider oriented website, many of which double as bunny supplies, so I tend to take for granted that I have just about anything I could ever need, lol.
Oh, and in regards to Critical Care... do NOT get the anise flavor! Get apple banana. GPs love anise, I've heard, but bunnies tend to hate it. Also, I shopped around for it last month; Amazon's got everyone else beat price-wise these days (I paid $8 for a bag w/free shipping).